This actually reminds me that there is also „lapte bătut“ in Bulgarian („бито мляко“, meaning is the same: ‘beaten milk’), and a similar trend for „iaurt de băut” („йогурт за пиене“, again the same ‘yoghurt drink’). I think it is the more literary variant of the „мътеница“-branch above since all those names for buttermilk there are from different regions.
I've seen that we have a lot of things, esp. foods or basic day-to-day phrases, that can literally be translated word for word and mean the same in Bulgarian–or South-Slavic in general–and Romanian, and often Albanian and/or Greek (and there are good historical reasons for that), so I was kind of surprised about how I interpreted the things I hastily found above. But thanks to you everything comes into place now. :)
Sour milk that you find in stores is usually a drinkable yoghurt that has more acidity than regular yoghurt. A good product would be a thick yoghurt drink with the consistence of a thick smoothie that's unevenly mixed and has a more acidic taste (much like ayran or kefir). Not to be confused with „iaurt de băut” (yoghurt drink) products which are a watered down variant of regular yoghurt with a boring, nondescript taste.